CMP is a semiconductor processing technology in which mechanical processing, which is performed using polishing particles present between a pressed wafer and a polishing pad, and chemical etching, which is performed using a slurry, are simultaneously conducted, and has been an essential process in global planarization technology in the production of submicron-scaled semiconductor chips since IBM Corp., USA, developed it at the end of the 1980's.
A description will be given of a CMP process and the slurry needed for this process, with reference to FIGS. 1a to 1c. The CMP process, in which an uneven portion of the surface of a wafer is smoothed in a semiconductor process, is a process where, after the surface of the wafer is chemically transformed by an acidic or basic solution of the slurry to instantaneously form a layer weakly bonded to the surface, the layer thus formed is mechanically removed using particles in the slurry. In other words, the wafer is pressed while the slurry is provided on the surface of the wafer, thereby mechanically polishing the surface of the wafer using the particles in the slurry.
To conduct the CMP process, a head 3, on which a wafer 1 is to be mounted, a pad 4, rotating in the same direction as the head, and a slurry 2, provided therebetween and containing nano-sized polishing particles, are prepared. The wafer 1 is mounted on a wafer chuck 7 of the head 3 by surface tension or vacuum pressure. In the CMP process, the wafer 1 is polished by the pad 4 and the slurry 2. A polishing table 5, to which the pad 4 is attached, only rotates, but the head 3 simultaneously rotates and reciprocates while deviating from the center of rotation of the polishing table 5. At the same time, the wafer 1 is pressed toward the polishing table 5 with a predetermined pressure. The surface of the wafer 1 comes into contact with the pad 4 due to the weight of the head and the applied pressure, and the slurry flows into fine gaps in the interface, that is to say, the pores 8 of the pad. Mechanical polishing is achieved by polishing particles of the slurry and surface protrusions 9 of the pad 4, and chemical polishing is achieved by chemical components of the slurry. Furthermore, upper sides of projections of the wafer 1, in which devices are formed, first come into contact with the polishing particles or the surface protrusions, and pressure is concentrated on the projections of the wafer. Accordingly, the projections are removed at relatively high surface removal speed, resulting in uniform removal of the projections.
The types of slurry are roughly classified into a slurry for oxide, a slurry for metal, and a slurry for poly-silicon according to the type of object to be polished. The slurry for oxide is used in a CMP process which is applied for a metal wiring layer formed in an integrated circuit, and as the fabrication of integrated circuits using a copper wiring layer has recently been suggested, studies on a slurry for Cu CMP are ongoing. Several hundred active devices formed in an integrated circuit are interconnected to form a functional circuit and are connected in a multi-level manner. Metal interconnects, including a first metal layer, a second metal layer and a third metal layer, are electrically insulated by a dielectric material such as silicon dioxide. In particular, as copper has been used as each of the metal wiring layers in an integrated circuit, and a low-k material has been used as an insulating material, the rate of occurrence of scratches in a CMP process has greatly increased. Although copper has an electrical conductivity higher than that of aluminum wiring and is highly advantageous for high integration, it is softer than other metals, and thus can be easily scratched. Also, because the low-k material which is used as an insulating material is a soft material, it is easily broken and damaged. For this reason, in polishing of the copper wiring metal and the insulating film, polishing particles having a spherical shape and a narrow particle size distribution must be used.
The prior fumed silica and ceria particles have a wide particle size distribution and an irregular shape, and thus have a problem in that these particles can cause numerous scratches when used to polish films made of soft materials such as copper and low-k material. To improve this problem, nanosized colloidal silica particles have recently been frequently used. The colloidal silica particles have a particle size distribution that is narrower than that of other particles, and are uniformly spherical in shape, and thus entail a very low rate of occurrence of scratches when used to polish the surface of films. However, the colloidal silica particles still cause scratches on the surfaces of a copper film and an insulating film, and need be improved to reduce the occurrence of scratches.
In the prior art for preparing such Cu-CMP slurry, Japanese Patent Publication No. JP-2000-00161519 (JSR Co., Ltd.; Japan) and U.S. Patent Publication No. 60/142,706 (Cabot Corp., USA) disclose polishers and methods for preparing slurries having a low rate of occurrence of scratches using the same. In these patent publications, the properties of particles required for Cu-CMP slurry, the kind of additives, including polymers, and a method and process for preparing Cu-CMP slurries using these additives, are described over a very complex and broad range.
Most inventions utilize soluble metal silicates in the synthesis of colloidal silica or silica particles, however, if the synthesis is performed using such metal silicates, it will be difficult to control the shape and particle size of colloidal silica, and the addition of impurities will be caused in subsequent semiconductor processing, because the metal silicates contain salts such as Na.
Also, the prior art discloses the kind of slurry particles and the control of scratches according to the range of the slurry particles or additives, and these prior preparation methods have a problem in that they can cause numerous micro-scratches according to CMP process conditions. Also, the prior art does not mention the modification of the particle surface or the effect thereof on scratches.